This invention relates generally to optical elements and more particularly to strengthening of optical elements.
As it is known in the art, optical imaging systems generally include one or more externally mounted optical elements which shield the remainder of the imaging system from an external environment. For example, with infrared airborne imaging systems, an IR transparent optical element, such as a window or dome, is generally mounted on the airborne system to isolate the remainder of the imaging system from exposure to humid, corrosive, and abrasive environments. Moreover, the element is also provided to isolate the remainder of the imaging system from a aerodynamic environment. Such elements often find applications on missiles, for example.
As system requirements for missile speed increase, the mechanical properties for domes and the like concomitantly increase.
Typically, materials which offer the best mechanical durability and optical performance for infrared imaging systems, in the long wavelength infrared imaging band between 8-12 microns, are limited to a relatively small number of materials. Suitable materials include Group II-VI materials, such as zinc sulfide, zinc selenide, mercury cadmium telluride, and cadmium telluride, Group III-V materials, such as gallium arsenide, gallium phosphide, and Group IV materials, such as germanium. Moreover, certain ternary sulfides having the general chemical formula MLn.sub.2 S.sub.4 where M is a Group I cation, Ln is a lanthanide rare earth series cation, and S is the S.sup.-2 sulfide anion are also being used or developed for IR applications in the 8-12 micrometer band. While these ternary sulfide materials have improvements in durability over the aforementioned materials, generally these materials are also relatively susceptible to aerodynamic loading, for example. One feature common to all of the materials is that they are relatively brittle and have relatively low mechanical strengths. Nevertheless, these materials are the best currently available for applications in the 8-12 micron band, principally because they provide the greatest degree of optical transparency.
In particular, zinc sulfide and zinc selenide are the materials of choice for applications requiring transmission in the 8-12 micron band. Although these materials are relatively weak, they have sufficient strength and high enough transmittance characteristics to make them the preferred choice for many of the aforementioned applications. That is, considering optical, physical, and mechanical properties of the materials, zinc sulfide and zinc selenide are currently materials of choice.
Attempts have been made to improve the mechanical properties of these LWIR materials. Such attempts to improve the materials have included providing thick and thin film coatings and compressive coatings of various materials over the LWIR materials, as well as implanting of species into the materials. These procedures have met with some success increasing the strength of these materials by up to approximately 40% and increasing the apparent fracture toughness of these materials by approximately 20%.
It has long been theorized that the actual strength of these materials is limited to a fraction of the theoretical strength principally by material flaws lying at or near the surface of the optical material. These flaws, commonly referred to as the "Griffith" flaw, when they lie near the surface of the substrate are susceptible to propagation through the substrate by tensile components of surface stress waves incident on the surface of the material. Once the flaw starts to propagate, its continued propagation through the material will produce a large crack which leads to a reduction in optical transparency and with sufficient propagation, leads to a catastrophic failure of the element.